Electrochemical conversion cells, commonly referred to as fuel cells, produce electrical energy by processing reactants, for example, through the oxidation and reduction of hydrogen and oxygen. A typical polymer electrolyte fuel cell comprises a polymer membrane (e.g., a proton exchange membrane (PEM)) with electrode layers (e.g., containing at a minimum one catalyst type and one ionomer type) on both sides. The catalyst coated PEM is positioned between a pair of gas diffusion media layers, and a cathode plate and an anode plate are placed outside the gas diffusion media layers. The components are compressed to form the fuel cell.
The catalyst layers can be attached to the PEM forming a membrane electrode assembly (MEA) (also known as a catalyst coated membrane (CCM)). One method of forming an MEA involves depositing an electrode ink on the PEM by direct spraying or coating in a shim frame. Alternatively, the electrode can be formed on a decal and transferred to the PEM. Alternatively, the catalyst/ionomer ink can be coated on a gas diffusion medium (GDM) substrate. This intermediate product is known as a catalyst coated diffusion media (CCDM).
Typically, the electrode ink includes powder catalyst on a support, such as a carbon support, and an ionomer solution which are dispersed in a mixed solvent. The mixed solvent usually contains one or more organic solvents, such as alcohols, and water in a specific ratio that depends on the type of catalyst. The mixture is then homogenized by ball-milling for up to about 3 days before coating on the PEM, decal substrate, or GDM. For shim coating, the catalyst loading can be controlled by the thickness of the shim; for the Mayer rod coating, the catalyst loading can be controlled by the thread number. Multiple coatings can be applied for higher catalyst loading, as needed. After applying the wet ink, the solvents are dried in an oven to drive off the solvent and form the electrode. After the catalyst/ionomer coated decal dries, the catalyst/ionomer is then transferred onto a PEM by hot press to form an MEA. The anode and cathode can be hot-pressed onto a PEM simultaneously. The pressure and time for the hot press may vary for different types of MEAs.
It is known that electrodes made from catalyst ink are prone to forming a network of cracks on the surface, which is called “mud cracking.” It is well known that stresses develop as a wet film dries and the solid materials begin to consolidate. Although not wishing to bound by theory, the cracks may form due to non-uniform drying of the catalyst ink during fabrication of the electrodes. The cracks may also form following drying due to an inherent weakness of the electrode. The electrode is formed from a porous matrix of the carbon support bound by the ionomer, which is a relatively weak binder. As a result, the matrix of the carbon support within the ionomer may not be continuous. In addition, the carbon support provides minimal reinforcement to the ionomer, and the resulting matrix may not withstand the substantial stresses during the drying of the catalyst ink, resulting in a greater opportunity for the cracks to form during operation of the fuel cell. If the tensile strength of the film is insufficient to overcome the induced drying stress, mud cracks can form to relieve the film of the stress.
The network of cracks can negatively impact the performance of the fuel cell in several ways. For example, during the typical expansion and contraction of the electrolyte membrane during fuel cell operation, the base of the cracks can form a stress concentration on the adjacent electrolyte membrane, which may result in degrading the membrane, for example, forming pin-holes. In addition, the electrolyte membrane immediately adjacent to the crack is exposed to a different humidity environment than the electrolyte membrane immediately adjacent to the electrode. The expansion of the electrolyte membrane into the cracks can also degrade the electrolyte membrane, particularly after repeated expansion and contraction cycles. Furthermore, the network of cracks in the electrode can reduce the effective stiffness of the electrode, resulting in an undesirable movement of the MEA during fuel cell operation.
A number of methods for reducing mud cracking have been developed. One method involves increasing the time allowed for the catalyst ink to dry. However, increasing the drying time also increases manufacturing costs and may not be sufficient to reduce cracking. Increasing the drying time also may not optimize the resistance to cracking of the electrode during subsequent operation of the fuel cell.
Therefore, there is a need for making an electrode which reduces mud cracking.